This invention relates to a waveguide-to-microstrip transition which provides broad bandwidth and which allows any desired orientation of the plane of the microstrip to the walls of the waveguide.
A transmission-line is a conductor arrangement for guiding electromagnetic waves from one location to another. One of the the simplest types of transmission-line is a two-wire or parallel-wire transmission-line, which consists of a pair of parallel wires which guide an electromagnetic wave propagating in the region around and between the wires. Such two-wire transmission-lines are inexpensive, effective and in very common use, but have certain disadvantages for use at microwave frequencies, which disadvantages include a relatively high degree of coupling between the transmission line and its environment, which allows undesired external noise signals to be added to the signal being propagated on the transmission-line, and the concomitant problem of leakage away from the transmission-line of the wave being guided which results in a propagation or transmission loss in excess of that due to resistive losses in the transmission line. Because of these problems, other types of transmission-line are more commonly used at microwave frequencies.
A coaxial transmission line (coax) is a tubular or pipe-like outer conductor which is coaxial with a central conductor or center-conductor. Coaxial transmission-lines confine the electromagnetic field in the region between the center conductor and the outer conductor. The outer conductor isolates the signal from the environment. Consequently, the signal being propagated thereon is relatively unaffected by the environment. Coaxial transmission lines are very popular, but have relatively high loss at the higher microwave frequencies.
Another type of transmission-line is the waveguide, which is a hollow conductive tube in which the electromagnetic energy is propagated without the benefit of a center-conductor. Waveguides have relatively narrow bandwidth, but within that bandwidth may have lower loss than coax.
Another type of transmission line is the so-called microstrip transmission line, which has a similarity to both two-wire transmission lines and to coax. A microstrip transmission-line consists of a relatively large ground plane from which a relatively narrow strip conductor is separated by a thin layer of dielectric material. Strip conductors are often manufactured by printing of the conductors onto plates of dielectric material. They are widely used for the interconnections in microwave active circuits such as amplifiers, circulators and the like.
It often happens that the need arises for coupling signals from waveguide to a TEM (transverse-electromagnetic) transmission-line such as coax or microstrip. This is normally accomplished by a transition. Several problems arise in design and manufacture of such transitions. One problem arises from the need to couple energy from one type of transmission line to the other without reflections caused by impedance mismatches. Such reflected energy results in less energy being propagated through the transition, and is therefore undesirable. It is generally possible by tuning to provide essentially zero loss at a single frequency. The difficulty arises when it is desired to couple energy with low loss over a broad range of frequencies (over a broad bandwidth). Another problem which arises lies in the orientation of the output transmission line relative to the input transmission-line. U.S. Pat. No. 3,478,282 issued Nov. 11, 1969 to Smith describes a waveguide-to-coax transition in which a stepped ridge provides broadband coupling between a waveguide and coaxial line. The coaxial line has its axis at right angles to the direction of propagation in the waveguide.
U.S. Pat. No. 3,737,812 issued June 5, 1973 to Gaudio et al describes a waveguide-to-coaxial transition having a stepped ridge for broadband impedance matching to a coax transmission-line, the axis of which lies in the same direction as the direction of propagation in the waveguide.
The article "Waveguide-to-Microstrip Transition Uses Evanescent Mode" by Bharj et al, published by Microwaves & RF Magazine, January 1984 describes a waveguide-to-microstrip transmission using a stepped ridge. This arrangement suffers from the problem of unwanted losses in the transition due to radiation of a significant amount of energy from the open end of the waveguide. This occurs because not all the energy is constrained within the region of ridge and is therefore not coupled to the microstrip. Also, the bandwidth tends to be restricted.
A broadband waveguide-to-coax or waveguide-to-microstrip transition is desired.